Preliminary Study of Single Particle Lidar for Wing Wake Survey. M.Valla, B. Augère, D. Bailly, A. Dolfi-Bouteyre, E. Garnier, M.

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1 Preliminary Study of Single Particle Lidar for Wing Wake Survey M.Valla, B. Augère, D. Bailly, A. Dolfi-Bouteyre, E. Garnier, M. Méheut

Context of research Clean Sky Joint Technology Initiative : Aims at reducing the environmental footprint of civilian aircraft & rotorcraft Smart Fixed Wing Aircraft (SFWA) objectives: 10 % reduction

2 Context of research Clean Sky Joint Technology Initiative : Aims at reducing the environmental footprint of civilian aircraft & rotorcraft Smart Fixed Wing Aircraft (SFWA) objectives: 10 % reduction of aircraft drag (25 % reduction of wing drag) 20 % reduction of fuel burn 10 db reduction of aircraft noise Part of ONERA work is to evaluate the feasibility of coherent lidar for non intrusive in flight wing drag measurements The following ONERA results have received funding from the European Community's Seventh Framework Program for the Clean Sky Joint Technology Initiative under grant agreement no SFWA CLRC 16 June 22 nd, 2011

3 Single Particle Lidar for Wing Wake Survey Lidar environment: Atmosphere: high altitude, clear sky aerosols Single particle coherent lidar Wing wind field: steady flight parameters, highly turbulent flow in the wing wake (standard deviation of wake turbulence: 10 to 15% of mean speed) Measurements characteristics objectives: Wing wake velocity profile: x component of flow velocity scanned along z axis Measurement location: approx. 1 chord down stream of the wing trailing edge Air velocity dynamic range: m.s -1 Air velocity accuracy objective 1 m.s -1 Particles Scan Laser beam Lidar pod Wing z x y 3 CLRC 16 June 22 nd, 2011

4 Lidar geometrical setup 3 stations with respect to the wing trailing edge Laser beam focused at range F T and orientated with θ angle compared to aircraft tail to nose direction in the XY plane Y Station 1 (1.5 chord); θ 1 =29.2 ; F T =5.6m Station 2 (1 chord); θ 2 =41.2 ; F T =4.1m Station 3 (0.5 chord); θ 3 =63.4 ; F T =3.04m Same span coordinate (24 m from center) θ 3 θ 2 θ 1 X Z Station3 Station1 Station2 For each station, the laser probe carries out a scan in the vertical Z axis to describe the Wind field Vx : γ scan =±8 ( station 2) Y Z X 4 CLRC 16 June 22 nd, 2011

5 Impact of lidar geometry Lidar velocity bias assessment: Lidar measurement = radial velocity, not x component of air velocity Non zero y and z component of air velocity Lidar measurement bias Higher bias at 0.5 chord, lower bias at 1.5 chord Simulations of lidar bias from 2.5D Elsa RANS (Reynolds-Averaged Navier Stokes) computed data x component of wind speed (m/s) Lidar measurement half chord z (m) Drag extraction assessment Fast Fluids Dynamics Code for RANS computed data 1st order and Jones formulations for Lidar data Vx Vx lidar x component of wind speed (m/s) Lidar measurement one and half chord z (m) Vx Vx lidar 5 CLRC 16 June 22 nd, 2011

6 Drag extraction : FFD / LIDAR comparison LIDAR Mach Fast Fluids Dynamics Code Cx 1st order Cx Jones x/c = 0,5 / 1,0 / 1,5 Transition 0, / 64 / / 61 / 60 0, / 54 / / 51 / 49 0, / 74 / / 71 / 69 Turbulent 0, / 94 / / 91 / 91 0, / 96 / / 93 / 92 0, / 106 / / 102 / 100 Turbulent - Transition 0,73 31 (31%) 33 (33%) / 30 (32%) / 29 (32%) 31 (33%) / 30 (33%) / 31 (34%) 0,75 46 (44%) 45 (44%) / 42 (44%) / 40 (44%) 43 (45%) / 42 (45%) / 43 (47%) 0,77 32 (28%) 34 (31%) / 32 (30%) / 28 (28%) 32 (31%) / 31 (30%) / 31 (31%) 6 CLRC 16 June 22 nd, 2011

7 FFD / LIDAR : Results Convergence of both experimental methods in the wake (1.5 chords) Maximum difference between CFD and experimental methods : 3% 7 CLRC 16 June 22 nd, 2011

8 Lidar efficiency Flow turbulence: Standard deviation of 40 m.s -1 in turbulent area (15% of mean flow speed inside the wake) Standard deviation of 5 m.s -1 in less turbulent area (out of wake) Single particle lidar measurement accuracy bounded by the flow turbulence: Lidar accuracy standard deviation of flow / square root of particles detected Lidar detection efficiency = Number of detection per second Aerosol density ranging from 1 to 5 part.cm -3 A priori fixed parameters: Laser power P L = 5W: available on the shelf, compact and its consumption is compatible with onboard operation Lidar footprint Beam radius 35 mm 8 CLRC 16 June 22 nd, 2011

9 Lidar efficiency without aero-optic effects Beam radius choice: Lidar detection efficiency = Number of detection per second Tradeoff between measurements at different chords : 0.5chord, 1chord, 1.5chord Optimal beam radius on pupil: 31mm 67 part/s at 0.5chord 75 part/s at 1 chord 64 part/s at particle per cm part/s at 0.5chord 375 part/s at 1 chord 320 part/s at 1.5 chords Ndet/s chord;Ft=4.1m;teta= chord;Ft=3m;teta= particle per cm E E E E E E E E E E E-02 Beam radius 1/e2 (m) 9 CLRC 16 June 22 nd, 2011

10 Preliminary evaluation of the aero-optic effects Air density fluctuations lead to refractive index fluctuations (Gladstone Dale law) n = 1 Kρ Expected effects: beam propagation in turbulent medium focused spot larger loss of lidar performance in terms of detection per second Preliminary assessment of aero optical effects required Two regions of interest Boundary layer on the pod Wake Flow conditions: altitude=40000 ft, Ma=0.77, turbulent case On the pod (boundary layer): a simple model = [ A( ρ ρ )] 2 σ 2 ρ wall ext Variance of the density: with A=0.2 Correlation length of the density fluctuation: = L ρ 0.1 layer thickness = 5 mm 10 CLRC 16 June 22 nd, 2011

Preliminary evaluation of the aero-optic effects On the wake: a k-ω stationary computation on a infinite wing Air density (direct output) Air density variance (from Aupoix formulae, only valided in

11 Preliminary evaluation of the aero-optic effects On the wake: a k-ω stationary computation on a infinite wing Air density (direct output) Air density variance (from Aupoix formulae, only valided in boundary layer) σ ρ = k Pr 2 t 2 dρ dz du dz Air density coherence length (assumed to be equal to wind coherence length reconstructed from a turbulence integral length scale with k and Ω ) Z dimension in m Air Density in kg.m X dimension in m Z dimension in m Air Density Variance in kg 2.m X dimension in m x Z dimension in m Air Density Coherence Length in m X dimension in m CLRC 16 June 22 nd, 2011

12 Preliminary evaluation of the aero-optic effects Objective: Finding an equivalent refractive index structure parameter C n2 (optical propagation of laser beam is well described with the knowledge of C n2 ) Assumptions: Air density is an isotropic random filed described with a spatial spectrum model Air density has a Von Kármán like spatial spectrum without inner scale Φ n 1 ( k ) = ( 2πL ) 2 2 K σ ρ Γ Γ(11 (3 2) Γ( ( k + 4π L ) Value of the Gladstone Dale constant K of m 3.kg -1, given at sea level pressure and temperature (no data given at flight level 400) Identification with Von Kármán spectrum yields: ˆ 2 C n ρ 6) 6) (2π ) K σ Γ(11 6) = sin( π 3) Γ(8 3) Γ(3 2) Γ(2 6) L CLRC 16 June 22 nd, 2011

13 Preliminary evaluation of the aero-optic effects From RANS computed data: Air density coherence length Outer scale of turbulence retrieved from 3D Fourier transform of spectrum Outer scale of turbulence & Air density variance value of refractive index structure parameter C n 2 Turbulent wing wake flow C n2 = m -2/3 highly turbulent medium Impact of refractive index turbulence on laser propagation proportional to lidar range: Higher aero optical penalty expected at 1.5 chord 13 CLRC 16 June 22 nd, 2011

14 Power budget modeling with aero-optic effects Aero optical effects: Worst case: entire optical path inside the wing wake Measurements at different chords : 0.5, 1 & 1.5 chord, impact of turbulence higher with distance At previous beam radius on pupil: 31mm 62 part/s at 0.5chord 37 part/s at 1 chord 0 part/s at particle per cm 3 N d e t /s chord;Ft=4.1m;teta= chord;Ft=3m;teta= chords;Ft=5.6m;teta= part/s at 0.5chord 185 part/s at 1 chord 0 part/s at particle per cm E E E E E E-02 Beam radius 1/e2 (m) 14 CLRC 16 June 22 nd, 2011

15 Conclusion A preliminary study of a single particle lidar for wing wake survey has shown two main issues: Biased x component of flow velocity due to geometrical constrains Presence of aero-optic effects due to the nature of the flow to be analyzed Bias assessment shown that lidar data are relevant for drag extraction Aero-optic effects estimated impact for measurements in the wing wake: Reasonable risk for measurements at 0.5 chord High risk for measurements at 1 chord No measurement possible at 1.5 chord Presence of uncertainties in the model (ex.: Gladstone Dale constant value) Single particle lidar for wing wake survey still has to go through a proof of concept phase 15 CLRC 16 June 22 nd, 2011

16 16 CLRC 16 June 22 nd, 2011

SPC Aerodynamics Course Assignment Due Date Monday 28 May 2018 at 11:30

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